Head-up display with narrow band reflective polarizer

ABSTRACT

A head-up display includes a projection system and a window having a target area where a reflective polarizer is positioned to reflect light from the projection system to a viewing area. Light from the projection system is p-polarized and strikes exposed window surface(s) at an acute angle to reduce or eliminate multiple or “ghost” images. The acute angle is closely matched to a Brewster angle of the exposed window surface(s). The reflective polarizer includes a multilayer stack with refractive indices of individual layers chosen to reflect p-polarized light substantially more than s-polarized light over a wide angular range that includes the acute angle. The reflective polarizer also can reflect infrared light to reduce cabin heating from solar radiation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/335,458, filed Dec. 31, 2002 now U.S. Pat. No. 6,952,312, nowallowed.

FIELD OF THE INVENTION

The present invention relates to projection displays that allow a userto see, displayed information while keeping vision in the generaldirection of other sources of information. The displayed information canbe reflected from a window, eyepiece component, or portion thereofthrough which the user can also view the other sources of information.

The term “Head-up display” (HUD) is used herein to refer to such displaysystems, whether employed in the window of a vehicle such as anaircraft, watercraft, or landcraft (including motor vehicles such asautomobiles, trucks, and motorcycles), in smaller scale systems such asgoggle lenses or helmet visors, or in other diverse applications.

BACKGROUND

A wide variety of HUD systems are known. Commonly, a projection systemis combined with a partial mirror (a partial reflector and partialwindow) as the final optical component for forming a projected imageviewable by the user. Simultaneously, the user can view other scenesthrough the partial mirror. The partial mirror is an important componentaffecting the usability of the display. Generally, the reflectivity ofthe partial mirror must be sufficient to reflect light from theprojector, but the partial mirror must also be sufficiently transparentto provide adequate viewing through it.

The partial mirror is typically placed on a windshield, canopy, or othertransparent substrate, generically referred to herein as a window.Alternatively, the bare surface of the window is sometimes used as thepartial mirror itself. In either case the combination of reflection fromthe partial mirror, and reflection from one or both of the inner andouter surfaces of the window, can produce a multiple image, or“ghosting” problem. This problem becomes more noticeable as the windowthickness increases and the line-of-sight through the window becomesmore oblique. For typical systems, ghosting problems become morenoticeable for window thicknesses on the order of about 1 mm and up.

One known solution to this problem is to wedge the inner and outersurfaces of the window so that the (predominantly s-polarized) lightreflecting off one surface is angularly separated from light reflectingoff of the partial mirror or opposite surface. However, adding a wedgeto a windshield can increase cost and manufacturing complexity. Further,achievable wedge angles for practical devices are very limited.

FIG. 1 depicts another known HUD system, disclosed in U.S. Pat. No.5,999,314 (Asakura et al.). The system, S, includes apolarization-direction changing layer 2 bonded to the inboard surface ofan outboard-side glass plate 1A which, together with an inboard-sideglass plate 1B and an intermediate film 4 (polyvinyl butyral or thelike), forms a laminated glass 5 used for an automotive front windshieldglass W. Light from a displaying device 6 passes through a polarizer 7so that the light is p-polarized (polarized in the plane of incidence).A Brewster's angle regulating film 3, made of TiO₂, is formed at theinboard surface of plate 1B. P-polarized light from the displayingdevice 6 is incident on laminated glass 5 at Brewster's angle (θ=63°) ofthe Brewster's angle regulating film 3. No reflection is made at theinboard surface of plate 1B because the p-polarized light is incident atBrewster's angle. The light that thus enters laminated glass 5 thenreaches polarization-direction changing layer 2, where the p-wave isrotated into an s-wave. The resulting s-polarized light is thenpartially reflected (about 20%) at an outboard surface of plate 1A. Thereflected s-polarized light is re-converted to p-polarized light uponpassing again through layer 2. This p-polarized light then passesthrough film 3 for observation by eyes 8 of the driver. The referencealso discusses forming an optional reflection film 9 (a thin film of Al,Au, Ag, or Cu) at the outboard surface of plate 1A to further increasereflectivity. By eliminating reflection of light from displaying device6 off the inboard surface of plate 1B, and by providing negligiblereflection from other interfaces seen in the figure, the referencestates that a double image cannot be formed.

One problem with the system of FIG. 1 is its reliance on reflection atan outer window surface, where water, ice, dirt, and the like cangreatly distort or impair the image quality. Another problem is itsreliance on a polarization-direction changing layer: such a layer thatworks well at large angles of incidence and over the entire visiblespectrum is difficult to make, and is not generally available.

Other known HUD systems project other types of polarized light towardthe viewer, such as s-polarized light or circularly polarized light.Generally, such systems suffer from the fact that some or all of theimage light directed toward the viewer is polarized in a horizontalplane, which is the very polarization component rejected by ordinarypolarized sunglasses. Thus if such sunglasses were used, the projectedimage would become substantially more difficult to see.

Thus, a need remains in the art for improved HUD systems in whichghosting is reduced or eliminated.

BRIEF SUMMARY

In brief summary, the present specification discloses HUD systems thatcomprise a window member, a display source, and a reflective polarizer.The window member has a target area and at least a first exposed windowsurface at the target area. A first normal axis is associated with theexposed window surface. The display source, which may itself include oneor more polarizers including e.g. an output polarizer, emitssubstantially p-polarized light toward the target area along anillumination axis. The illumination axis makes an acute angle θ₁ withrespect to the first normal axis. The reflective polarizer is disposedproximate the target area, and reflects at least some of the p-polarizedlight from the display source towards a viewing position. Notably, thereflective polarizer has a higher reflectivity for p-polarized lightthan for s-polarized light over an angular range that includes the angleθ₁. In preferred embodiments the angular range is desirably at leastabout 20°, and more desirably at least about 40°, 60°, or more.

The reflective polarizer desirably is substantially transparent for thes-polarized component of the certain obliquely incident light, and is atleast partially reflective for the p-polarized component. For example,the reflectivity for p-polarized light is at least about 20% and moredesirably at least about 40% and the reflectivity of s-polarized lightis less than about 10% and more desirably less than about 5% over theangular range.

Moreover, the light incident on the polarizer can comprise some or allof the visible portion of the electromagnetic spectrum. It can alsocomprise infrared wavelengths. Alternatively, the reflective polarizercan be configured such that both s-and p-polarized light, at near-normalincidence and at oblique angles, are highly reflected in the infraredregion, in which case additional rejection of solar radiation forexample can be achieved.

Additional and/or alternative features are set forth with moreparticularity below.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the specification reference is made to the appended drawings,where like reference numerals designate like elements, and wherein:

FIG. 1 is a fragmentary schematic sectional view of a PRIOR ART displaysystem;

FIG. 2 is a fragmentary schematic sectional view of a HUD system asdescribed herein;

FIG. 3 depicts various obliquely incident light rays impinging on apartial reflector;

FIGS. 4 a–b are simplified, idealized graphs showing the qualitativebehavior of light reflectivity versus angle of incidence (in air), inthe x-z plane and the y-z plane respectively, for a simple metal coatingthin enough to act as a partial mirror;

FIGS. 5 a–b are graphs similar to FIGS. 4 a–b but for a multilayerreflective polarizer wherein for alternating layers Δn_(x) is large,Δn_(y) is substantially zero, and Δn_(z) is large and of the samepolarity as Δn_(x);

FIGS. 6 a–b are graphs similar to FIGS. 5 a–b but for a multilayerreflective polarizer wherein for alternating layers Δn_(x) is large,Δn_(y) is substantially zero, and Δn_(z) is zero or very small comparedto Δn_(x);

FIGS. 7 a–b are graphs similar to FIGS. 6 a–b but for a multilayerreflective polarizer wherein for alternating layers Δn_(x) is large,Δn_(y) is substantially zero, and Δn_(z) is large but of oppositepolarity to Δn_(x);

FIGS. 8 a–b are graphs similar to FIGS. 7 a–b but for a multilayerreflective polarizer wherein for alternating layers Δn_(x) and Δn_(y)are both substantially zero, but Δn_(z) is large;

FIG. 9 is a computed graph of transmissivity versus wavelength for abroadband reflective polarizer, for one linear polarization component atnormal incidence;

FIG. 10 is a plot showing the layer thickness profile of a multilayerthin film stack capable of producing the graph of FIG. 9;

FIG. 11 is a computed graph of transmissivity versus wavelength foranother broadband reflective polarizer, for one linear polarizationcomponent at normal incidence; and

FIG. 12 is a plot showing the layer thickness profile of a multilayerthick film stack capable of producing the graph of FIG. 11.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

A partial schematic view of an illustrative HUD system 10 is shown inFIG. 2. In brief, a projection system 12 is provided to direct light 14towards a target area 16 of a window 18. A reflective polarizer 20,disposed proximate the target area 16, then reflects at least some ofthe projected light towards the intended viewer 22. Simultaneously, thereflective polarizer 20 and the window 18 transmit at least some light24 from the outside environment to permit observation thereof by viewer22.

Projection system 12 can be a conventional system that projects avisible light beam or image, and can include known elements such as anLCD, electroluminescent panel, incandescent or phosphorescent lightsource, CRT, LEDs, and lenses, collimators, reflectors, and/orpolarizers. The emitted light can be substantially monochromatic,polychromatic, narrow band, or broad band, but preferably overlaps atleast a portion of the (visible) spectrum from about 400 to 700 nm.Significantly, the light 14 emitted towards target area 16 issubstantially linearly polarized in the plane of the figure as shown.While it is understood that system 12 will emit light over a finiteangular cone, only one ray of light 14 is depicted for ease ofillustration. Furthermore, system 12 can also include a mechanism suchas a tilting mirror or displacement means to change the angle and/orposition of emitted light so as to accommodate viewers at differentpositions or heights.

Light 14 travels along an illumination axis 26 and strikes a firstexposed major surface 18 a of window 18. As suggested previously, window18 can comprise any of a wide variety of transparent members, and can beunitary or laminated, flat or curved (simple or compound curvature),water clear or tinted, can have focusing properties (e.g. in the case ofgoggles or other eyewear), and can be composed of any conventionalglasses and/or plastics. For low cost and weight, the window cancomprise a sheet of glass or other transparent material with planeparallel surfaces. As shown, the polarized light 14 strikes surface 18 aat an acute angle θ₁ with respect to an axis 28 that is normal tosurface 18 a at the point of entry. The normal axis 28 and illuminationaxis 26 define a plane of incidence that is coincident with the plane ofthe figure. Light 14 is polarized in the plane of incidence and thus isdesignated “p-polarized” light. As is known, ordinary optical materialssuch as glass and plastic exhibit a Brewster angle θ_(B) in air that isa function of the refractive index of the particular optical material,from roughly 55 to 60 degrees for typical window materials. Althoughstrictly speaking θ_(B) is a function of optical wavelength due todispersion, such effects are typically very minor and in most casesθ_(B) can be treated as constant over the visible spectrum. For lightincident at the Brewster angle, any p-polarized component has zeroreflectivity, while any s-polarized component (whose electric fieldvector is perpendicular to the plane of incidence) has a reflectivityeven greater than its reflectivity at near normal incidence. P-polarizedlight incident at angles near θ_(B) experience nonzero but very lowreflectivity. Therefore, by keeping incident angle θ₁ equal to or closeto θ_(B), and ensuring that light 14 has little or no s-polarizationcomponent, no significant reflection of projection light 14 takes placeat surface 18 a. This avoids a “ghost” image being created at surface 18a. The degree to which θ₁ can deviate from θ_(B) while still maintaining“no significant reflection” depends on many system variables andrequirements. Generally, deviations of at least a few degrees of arc aregenerally believed permissible. In some cases deviations of greater than5 or even 10 degrees may be acceptable, since p-pol reflectivity isstill typically much reduced at such angles compared to reflectivity atnear-normal incidence, particularly for angles less than θ_(B). Notethat some angular deviations from θ_(B) are unavoidable to the extentprojection system 12 is uncollimated, target area 16 is extended, andsurface 18 a is non-flat.

Depending on the characteristics of reflective polarizer 20, some lightfrom system 12 propagates onward to a second exposed major surface 18 bof window 18. For some embodiments, surface 18 b will be substantiallyparallel to surface 18 a, and local normal axis 30 will be parallel tonormal axis 28, and the Brewster angle for surface 18 b will be the sameas the Brewster angle for surface 18 a—but these conditions will not ingeneral be satisfied. Where the conditions are satisfied, or nearlysatisfied, light 14 will exit window 18 at an exit angle θ₂ equal to θ₁and will again experience no significant reflection due to the effectiveabsence of s-polarized light and propagation at or near Brewster'sangle. Hence, another “ghost” image is avoided.

In some embodiments it may be desirable or necessary to provide anantireflection coating on surface 18 a and/or surface 18 b. Suchcoatings are easier to fabricate for p-polarized light at high incidenceangles (angles of at least about 40 degrees in air) than for s-polarizedlight at such angles. Further, Brewster angle regulating films asdiscussed in '314 Asakura et al. can also if desired be provided on theexposed window surfaces to make modest adjustments to θ_(B) in order tomaintain low reflectivity at surfaces 18 a, 18 b. But for manyembodiments antireflection coatings and Brewster angle regulating filmscan be completely avoided.

An ideal HUD system would provide substantial reflectivity for lightfrom the imaging system while simultaneously providing high transmissionfor light coming from the external environment. For this reason, HUDsystem 10 incorporates reflective polarizer 20. Polarizer 20 can providesubstantially complete transmission of one polarization of light(s-polarization) while providing selective reflection and transmissionof the other polarization (p-polarization). Polarizer 20 preferablycomprises a multilayer stack of alternating materials such as describedin U.S. Pat. No. 5,882,774 (Jonza et al.), incorporated herein byreference. The polarization-specific reflection properties can be madewavelength specific by appropriate layer thickness selection, so thatthe polarizer is substantially transparent for all but the selectedwavelength(s), where it becomes reflective for only p-polarized light.The selected wavelength(s) can be a single narrow band, multiple narrowbands, or a broad band. Any suitable degree of reflectivity for thechosen band of wavelengths can be achieved by control of thelayer-to-layer refractive index differences and the total number oflayers in the polarizer. Polarizer 20 can be fabricated from tens orhundreds of co-extruded polymer layers that are substantiallynonabsorbing in the visible and near infrared wavelengths, such that thesum of reflectivity and transmissivity for the polarizer is 100%.

For purposes of the present application, unless otherwise indicated,reflectivities and transmissivities referred to in connection with thepolarizing film do not take into account any Fresnel reflection that mayoccur at the outermost surfaces of the polarizing film. Thus,measurements on a sample of polarizing film in air will typically yieldhigher reflectivities and lower transmissivities as a result of suchFresnel reflections, unless, for example, the film is provided withanti-reflection coatings on its outer surfaces.

Appropriate control of the so-called z-index difference (i.e., therefractive index difference along an axis perpendicular to the plane ofthe polarizer) for adjacent layers in the stack can eliminate theBrewster angle for the polarizer, providing highly efficient reflectionof p-polarized light. Such reflection can also be effective over a wideangular range to permit the same polarizing film to be useful in avariety of HUD systems having different incidence angles θ₁, and/or forgreater design flexibility. For instance, polarizer 20 need not beparallel to surface 18 a or 18 b. This would permit an exit angle θ₃(measured from axis 28) for light 14 a to differ from incidence angleθ₁.

Although typically the reflective polarizer will extend over only asmall portion of a window—which portion would ordinarily correspond tothe target area 16—the reflective polarizer in some embodiments canextend in a continuous strip across the entire window 18 or can even befully coextensive with window 18. For motor vehicles, the strip canextend horizontally along a lower, upper, or intermediate portion of thewindshield. Depending on the size of the window and the application, thetarget area can likewise extend over only a small portion, a strip, orthe entire window.

One benefit of HUD system 10 is compatibility with polarized sunglasses.For many motor vehicle applications, glare from the outside environmenttends to be polarized along a horizontal axis. This glare is representedby reference numeral 32 in FIG. 2. In contrast, the portion 14 a oflight from the projection system reflected by polarizer 20 toward theviewer 22 is polarized along an axis lying in a vertical plane.Conventional polarized sunglasses selectively transmit verticallypolarized light and block horizontally polarized light, thus blockingglare 32 but transmitting the HUD reflected light 14 a. Note that thesunglasses will also block the horizontal polarization component ofdesirable outside environment light 24. The reflective polarizer 20 inthis case should not reflect all visible p-polarized light because verylittle of the desired outside environment light 24 would be seen by asunglass-wearing viewer 22 through target area 16.

We turn now to FIG. 3, where a partial reflector 40, or a locally flatportion thereof, is shown lying in an x-y plane of an x-y-z Cartesiancoordinate system. This reference system will be used in the discussionthat follows to describe the properties and particularly the advantagesof certain kinds of partial reflectors relative to others for the HUDsystem 10. Light rays are shown impinging obliquely on the reflector,with some rays lying in the x-z plane and others lying in the y-z plane.Light in either of these planes can have both an s- and p-polarizationcomponent. In cases where partial reflector 40 is a polarizer, they-axis is designated as the pass-axis, and the x-axis is designated atthe blocking or reflective axis. The angle of incidence θ is the acuteangle between the direction in air of the incident light ray and thez-axis, which is orthogonal to partial reflector 40.

FIGS. 4–8 are presented for comparison purposes. The curves in thesefigures depict the general qualitative reflectivity characteristics ofs- and p-polarized light as a function of incidence angle for certainpartial reflectors, and they are not taken from real or even computeddata. However, they are believed sufficiently accurate for comparisonpurposes. The curves assume a wavelength of light within the desiredspectral band as discussed above.

In FIGS. 4 a and 4 b, the reflectivity properties of a simplehalf-“silvered” mirror are shown for light incident in the x-z and y-zplanes respectively. Such a partial reflector can be made of a singlethin layer of silver, aluminum, gold, or other conventional metaldisposed on a transparent substrate. The metal thickness is such that50% of normally incident light is reflected. Note that transmission atnormal incidence will be less than 50% since metals generally absorbvisible light. The two figures are identical because the metal film isisotropic. As θ increases, reflectivity for s-polarized light increases.Reflectivity for p-polarized light dips somewhat, then increases.Although the p-pol reflectivity is quite high over a wide angular rangeof oblique incidence angles, including typical Brewster angles for glassand plastic, such a partial reflector will not work optimally in HUDsystem 10 because it would be difficult to see the outside environmentthrough such a reflector. This is chiefly due to the s-pol reflectivitybeing on the order of or higher than the p-pol reflectivity throughoutthe angular range. Reducing the thickness of the metal layer canincrease the transmission of the partial mirror to increase thevisibility of the outside environment, but at the expense of reducingthe brightness of the projected image.

FIGS. 5 a and 5 b show the reflectivity properties for a multilayerbirefringent reflective polarizer for light incident in the x-z and y-zplanes respectively. The refractive index relationships between adjacentlayers in the multilayer stack are:Δn _(x)≈large (+)Δn_(y)≈0Δn _(z)≈large (+)This is referred to as a “z-mismatched” polarizer. The “+” for Δn_(x)and Δn_(z) indicates those differentials are of the same polarity—thelayer having the higher x-direction refractive index n_(x) also has thehigher z-direction refractive index nz, and vice versa. “Large” forΔn_(x) means a refractive index difference sufficient to reflect onepolarization of normally incident light by at least about 20%, and morepreferably about 50% or more, for a given number of layers in the stack.For known birefringent polymer stacks this would typically be at leastabout 0.1, or at least about 0.2. Zero for Δn_(y) means a refractiveindex difference small enough to reflect a negligible amount, preferablyless than 10% or 5%, of the orthogonal polarization of normally incidentlight, for a given number of layers in the stack. For known birefringentpolymer stacks this would typically be at most about 0.02, or 0.01.“Large” for Δn_(z) means on the order of Δn_(x).

As shown by the figures, the z-mismatched polarizer has some p-polreflectivity in both the x-z plane and the y-z plane at oblique angles.In the y-z plane, p-pol reflectivity can be substantial over a wideangular range. But unfortunately the s-pol reflectivity is also verylarge over the same range. In the x-z plane, p-pol reflectivity isgreater than s-pol reflectivity over a wide angular range, except wherethe former becomes zero due to Brewster angles effects in the multilayerstack itself. Moreover, the p-pol reflectivity changes rapidly as afunction of incidence angle near the zero reflectivity point. The highs-pol reflectivity in the y-z plane, and the Brewster angle effect andrapid fluctuations of p-pol reflectivity in the x-z plane, mean that thez-mismatched polarizer also will not work optimally as a partialreflector in HUD system 10.

FIGS. 6 a and 6 b show the reflectivity properties for a differentmultilayer birefringent reflective polarizer for light incident in thex-z and y-z planes respectively. The refractive index relationshipsbetween adjacent layers in the multilayer stack are:Δnx≈largeΔny≈0Δnz≈0This is referred to as a “z-matched” polarizer. “Large” for Δn_(x) andzero for Δn_(y) mean the same as in FIGS. 5 a–b. Zero for Δn_(z) means arefractive index difference whose absolute value is at least less thanhalf of Δn_(x), and more preferably less than one-fourth or one-tenth ofΔn_(x).

As shown in FIG. 6 a, p-pol reflectivity for light in the x-z plane isrelatively constant and substantially higher than s-pol reflectivityover a wide range of incidence angles, indeed over the entire range ofincidence angles. The p-pol reflectivity decreases slightly withincreasing θ for small Δn_(z) values of the same polarity as Δn_(x), andincreases slightly with increasing θ for small Δn_(z) values of theopposite polarity as Δn_(x). This behavior is shown by the broken lines.The s-pol reflectivity is negligible—well below 10%, and well below5%—over the entire range of incidence angles. In contrast, lightincident in the y-z plane experiences high and increasing s-polreflectivity with angle, and negligible p-pol reflectivity over theentire range of incidence angles. Clearly, the z-matched reflectivepolarizer is well suited as a partial reflector in HUD system 10. But asdemonstrated by the differences between FIGS. 6 a and 6 b, the polarizermust be oriented properly for best performance. Preferably, the x-zplane coincides with the plane of FIG. 2. In motor vehicle applications,the pass-axis (y-axis) of the polarizer is preferably horizontal.

FIGS. 7 a and 7 b show the reflectivity properties for a differentmultilayer birefringent reflective polarizer for light incident in thex-z and y-z planes respectively. The refractive index relationshipsbetween adjacent layers in the multilayer stack are:Δn _(x)≈large (+)Δn_(y)≈0Δn _(z)≈large (−)This is referred to as a “negative z-mismatched” polarizer. “Large” forΔn_(x) and zero for Δn_(y) mean the same as in FIGS. 6 a–b. “Large” forΔn_(z) means having a magnitude of the same order as Δn_(x). The “+” and“−” signs indicate that Δn_(x) and Δn_(z) are of opposite polarity: thelayer having the higher value of n_(x) has the lower value of n_(z), andvice versa.

The reflectivity behavior of the negative z-mismatched polarizer is thesame as that of the other z-mismatched polarizer for light in the y-zplane (compare FIGS. 7 b and 5 b). But for light in the x-z plane, thep-pol reflectivity is not only maintained but increases with angle asshown in FIG. 7 a, while s-pol reflectivity remains negligible over allincidence angles. If desired, the total number of layers in the stackcan be reduced and/or the magnitude of Δn_(x) can be reduced to lowerthe p-pol reflectivity curve in FIG. 7 a and increase or eliminate theangle at which the p-pol reflectivity reaches saturation. Whether or notthis is done, the negative z-mismatched reflective polarizer provideshigh p-pol reflectivity and negligible s-pol reflectivity over a wideangular range, indeed over all angles of incidence for light in the x-zplane, making it generally well suited for use in the HUD system 10.However, the substantial increase of p-polarized light with θ may bedisadvantageous in some applications.

FIGS. 8 a and 8 b show the reflectivity properties for still anothermultilayer birefringent reflective polarizer for light incident in thex-z and y-z planes respectively. The refractive index relationshipsbetween adjacent layers in the multilayer stack are:Δn_(x)≈0Δn_(y)≈0Δn_(z)≈largeThis is referred to as an “off-axis” polarizer. Zero for Δn_(x) and forΔn_(y) mean sufficiently small to produce a negligible amount of on-axis(θ=0) reflectivity for either polarization, e.g. less than about 10% orless than about 5%. “Large” for Δn_(z) means large enough to produce adesired substantial amount of off-axis reflectivity. A preferred valuefor Δn_(z) is about 0.1 or greater. Reference is made to commonlyassigned U.S. patent application Ser. No. 10/334,836, entitled“P-Polarizer With Large Z-axis Refractive Index Difference”, and U.S.patent application Ser. No. 10/335,460, entitled “Optical PolarizingFilms With Designed Color Shifts”, both incorporated herein byreference.

Note initially that FIGS. 8 a and 8 b are identical because ofsubstantial symmetry of the off-axis polarizer in the plane of the film(the x-y plane). Beyond that, s-pol reflectivity is negligible over awide angular range, indeed over all incidence angles. To the extent thein-plane index refractive indices are not exactly matched, there will bea small amount of s-pol reflectivity that increases with θ. But note howthe p-pol reflectivity increases rapidly with increasing θ. The brokenlines are provided to show that the curve can rise more rapidly withangle for larger values of Δn_(z) (and/or more layers in the stack), andmore slowly with angle for smaller values of Δn_(z) (and/or fewer layersin the stack). Of course, further reductions can prevent the p-polreflectivity from ever reaching saturation. In any event the off-axispolarizer can provide a substantial amount of p-pol reflectivity and aninsubstantial amount of s-pol reflectivity over a wide angular range.Advantageously, this type of polarizer, and to a lesser extent thenegative z-mismatched polarizer, provides higher levels of p-polreflectivity at greater angles of incidence where the HUD incidenceangle θ₁ is more likely to be, and is more transparent at near-normalincidence. The off-axis polarizer is substantially clear at normalincidence. Another significant advantage specific to the off-axispolarizer is the ability to position it in the HUD system without regardto the polarizer orientation due to its in-plane symmetry. However, asdiscussed previously, the substantial increase in p-pol reflectivitywith angle may be disadvantageous in some applications.

The reader's attention will now be turned to desirable spectralproperties of suitable reflective polarizers. These properties areinfluenced not only by the refractive indices of the alternating layersin the multilayer stack, but also importantly by the layer thicknessdistribution throughout the stack. Also, these properties can changewith angle of incidence.

Various reflective polarizer designs can be utilized for partialtransmission of p-polarized light through the partial reflector for theHUD system. Two of these are: (1) colored polarizers that reflect onlyselected visible wavelengths of p-polarized light, and (2) broadbandpolarizers that reflect all visible wavelengths of p-polarized lightuniformly, but at an intermediate level of reflectivity such as 25% or50%. In the first type, the reflectivity within the band can be chosento be up to 100%. It is understood that the reflection bands discussedhere take into consideration any spectral shifts caused by the angularposition of the light source with respect to the polarizer. That is, thereflection bands discussed here are the reflection bands of thepolarizer at the intended use angle, not necessarily at normalincidence, unless otherwise specified.

One example of the first type of spectral properties is a reflectivepolarizer that exhibits a multiplicity of reflection bands separated byintervening transmission bands. For example, a cholesteric or thin-filmreflective polarizer can be designed to reflect three relatively narrowwavelength bands in the red, green, and blue portions of the visiblespectrum. As is well known, red, green, and blue light can be added invarying combinations to produce essentially any color of the spectrum.In this way, a full-color HUD can be created without the use of apolarizer that reflects the entire visible spectrum. Blue, green, andred reflection bands can be provided in the reflective polarizer in thespectral ranges between about 400–480 nm, 500–570 nm, and about 600–700nm respectively. To maximize transmission efficiency, only a narrowportion of each of these ranges is reflected by the polarizer,preferably about 50 nm or less in width (full width at half maximum),more preferably about 10–40 nm in width, and most preferably about 10–30nm in width. Reference is made to U.S. Pat. No. 6,157,490 (Wheatley etal.), incorporated herein by reference, for layer thickness distributiontechniques that can provide sharp bandedges for narrow reflection bandswith high maximum reflectivity values. If the display source emits lightin three (or less than three) narrow bandwidths, then the narrowreflection bands of the polarizer at the intended incidence angle(s)overlap the emission wavelength(s) of the source. In some embodimentsthe polarizer can provide only one or two reflection bands in thevisible region. Furthermore, the polarizer can also comprise one or moreinfrared reflection bands which, in vehicle applications, can reducesolar heating of the cabin environment.

Another example utilizing the first type of spectral properties is a HUDsystem wherein the reflective polarizer has a reflectance band locatedsubstantially entirely in the infrared range (which range includes thenear infrared range) at normal incidence, but which band is shifted intovisible wavelengths, typically red wavelengths, when incident at thechosen incidence angle. As a result, the reflective polarizer reflectsred (and transmits cyan, the complement of red) p-polarized light at theuse angle. In addition to its usefulness in a HUD, one advantage of sucha reflective polarizer is that it can provide a cooling function byreflecting infrared radiation of at least one polarization away from theviewer. Reflective polarizers useable with this embodiment can becholesteric reflective polarizers and thin-film reflective polarizers. Acharacteristic of both cholesteric and thin-film reflective polarizersis that the reflectivity spectrum shifts to shorter wavelengths withincreasing angle of incidence θ. In this embodiment the display sourceprovides light—e.g., red light—within the shifted wavelength range.Reflectivity in the red can be tailored so that a sufficient amount ofred light is transmitted through the target area of the window to enablethe viewing of external red objects such as traffic lights. The latteris of concern only if the viewer is also wearing polarized sunglassesthat would block substantially all of the orthogonally polarized (i.e.,horizontally polarized) red light.

Since cholesteric polarizers reflect circularly polarized light, aquarter-wave plate is used to convert linear s- and p-polarized light tocircular. The off-angle characteristics of the quarter-wave plate may beoptimized by controlling the relative values of the x-, y-, andz-indices of the film.

For a full color HUD system that shows no coloration in the reflector inany lighting situation, it is advantageous to use a broadband reflectivepolarizer. The level of reflectivity can be adjusted such that anytransmission requirement for the window is met, while still providing ahigh reflectance for the projected light. For example, a minimum averagetransmission of about 70% is required for front windshields of somemotor vehicles. A broadband reflecting polarizer can thus be made withthe transmission spectrum for one linear polarization at normalincidence shown in FIG. 9 and the orthogonal linear polarization havingessentially 100% transmission. Such a film has approximately a 75%average transmission for unpolarized light. At the same time, since thesum of reflectivity and transmission for the preferred polarizers issubstantially 100%, about 50% of the polarized projector light isdirected towards the viewer. A z-matched thin-film polarizer havingthese spectral properties can be made with 224 individual layers havingthe layer thickness profile shown in FIG. 10. The points fall along twocurves. The lower curve is for a first alternating material (e.g.polyethylene terephthalate (PET)) having a relatively high n_(x)refractive index of 1.68. The upper curve is for a second alternatingmaterial (e.g. a copolyester) having a relatively low n_(x) refractiveindex of 1.54. The y- and z-refractive index for all layers is 1.54.This example uses a graded quarter-wave stack design where the opticalrepeat unit within the multilayer stack consists essentially of twoadjacent layers, where the total optical thickness of a given opticalrepeat unit equals half the optical reflected wavelength.

Other known stack designs, e.g. those using more than two layers peroptical repeat unit, are also contemplated. In one such design,disclosed in U.S. Pat. No. 5,360,659 (Arends et al.), each opticalrepeat unit has six layers in which two alternating materials arearranged with relative optical thicknesses of about 7-1-1-7-1-1. Inanother design, disclosed in U.S. Pat. No. 5,103,337 (Schrenk et al.),each optical repeat unit has four layers in which three materials A, B,C, whose refractive indices are such that n_(A)>n_(B)>n_(C) and wheren_(B)=√(n_(A)n_(C)), are arranged as ABCB with relative opticalthicknesses of ⅓, ⅙, ⅓, ⅙ respectively. Hybrid film designs that utilizemore than one of the quarter-wave construction, the 7-1-1-7-1-1construction, and the ABCB construction are also contemplated.

An alternative approach to achieve broad band reflection is to usesignificantly thicker layers in the multilayer optical stack. Thetransmission spectrum shown in FIG. 11, for example, can be achieved ina z-matched polarizer using 124 thicker layers as shown in FIG. 12. Thelower curve in FIG. 12 is for a first alternating material (PEN) havingn_(x)≈1.85. The upper curve is for a second alternating material (PETG,available from Eastman Chemical Company) having n_(x)≈1.565. The y- andz-indices are all matched at 1.565, yielding a colorless transmission atall angles for the pass polarization. These indices can be achieved withPEN that is uniaxially oriented in a manner that allows the film tocontract in the non-stretch direction. Note that this film has anapproximately 50% reflectivity extending from 400 to 1800 nm for thep-polarization, thus reflecting approximately 25% of all solarradiation. This level of reflectivity will remain constant withincreasing θ because of the z-index match.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the scope and spiritof this invention, and it should be understood that this invention isnot limited to the illustrative embodiments set forth herein.

1. A system for displaying information to a viewing position,comprising: a window member having a target area and at least a firstexposed window surface at the target area, the first exposed windowsurface having associated therewith a first normal axis; a displaysource that emits polarized light toward the target area along anillumination axis that makes an acute angle θ₁ with respect to the firstnormal axis, the emitted light being substantially p-polarized andincluding a selected visible wavelength of interest; and a reflectivepolarizer disposed proximate the target area to reflect at least some ofthe p-polarized light from the display source towards the viewingposition; wherein the reflective polarizer reflects the p-polarizedlight in a narrow reflection band that includes the selected visiblewavelength of interest over an angular range of incidence angles thatincludes the angle θ₁.
 2. The system of claim 1, wherein the selectedvisible wavelength of interest is one of a plurality of selected visiblewavelengths.
 3. The system of claim 2, wherein the narrow reflectionband is one of a plurality of narrow reflection bands and wherein atleast some of the reflection bands correspond to some of the selectedvisible wavelengths.
 4. The system of claim 1, wherein the narrowreflection band is one of a plurality of narrow reflection bands.
 5. Thesystem of claim 1, wherein the narrow reflection band is 10–40 nm inwidth.
 6. The system of claim 5, wherein the narrow reflection band is10–30 nm in width.
 7. The system of claim 1, wherein the reflectivepolarizer is substantially transparent for wavelengths outside thenarrow reflection band.
 8. The system of claim 1, wherein the narrowreflection band is located substantially entirely in the infrared rangefor light incident along the normal axis, and wherein the narrowreflection band shifts into visible wavelengths for light incident atthe angle θ₁.
 9. The system of claim 1, wherein the reflective polarizerprovides substantially complete transmission for s-polarized light andwherein the reflective polarizer provides selective reflection andtransmission of p-polarized light.
 10. The system of claim 9, whereinthe selective reflection and transmission of p-polarized light includesa plurality of narrow reflection bands separated by interveningtransmission bands.
 11. The system of claim 9, wherein the selectivereflection of p-polarized light includes a blue reflection band, a greenreflection band, and a red reflection band for light incident at angleθ₁.
 12. The system of claim 11, wherein each of the blue, green, and redreflection bands is 10–40 nm in width.
 13. The system of claim 1,wherein the reflective polarizer is a cholesteric reflective polarizer.14. The system of claim 1, wherein the reflective polarizer is a thinfilm reflective polarizer.